Rail-based modular disc prosthesis

ABSTRACT

A method and apparatus for repairing a damaged intervertebral disc nucleus in a minimally invasive manner utilizes a modular disc prosthesis preferably comprised of at least three modular segments and at least two rails that operably connect adjacent modular segments. In one embodiment, each modular segment includes a harder inner portion and a softer outer portion. Preferably, the rails operate to slidably connect and interlock adjacent modular segments. A stem portion of the rails that extends outside the periphery of the body of the prosthesis is removable after implantation such that the modular segments form an implanted unitary device that closely mimics the geometry of the disc nucleus cavity.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 11/372,477, filed Mar. 9, 2006, which also claims the benefit of U.S. Provisional Patent Application No. 60/685,332, filed May 24, 2005, U.S. Provisional Patent Application No. 60/700,459, filed Jul. 19, 2005, and U.S. Provisional Patent Application No. 60/660,107, filed Mar. 29, 2005, the disclosures of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to an implantable prosthesis for repairing damaged intervertebral discs. More particularly, the present invention relates to a rail-based modular disc prosthesis of predetermined size and shape.

BACKGROUND OF THE INVENTION

The spinal motion segment consists of a unit of spinal anatomy bounded by two vertebral bodies, including the two vertebral bodies, the interposed intervertebral disc, as well as the attached ligaments, muscles, and the facet joints. The disc consists of the end plates at the top and bottom of the vertebral bones, the soft inner core, called the nucleus and the annulus fibrosis running circumferentially around the nucleus. In normal discs, the nucleus cushions applied loads, thus protecting the other elements of the spinal motion segment. A normal disc responds to compression forces by bulging outward against the vertebral end plates and the annulus fibrosis. The annulus consists of collagen fibers and a smaller amount of elastic fibers, both of which are effective in resisting tension forces. However, the annulus is not very effective in withstanding compression and shear forces.

As people age the intervertebral discs often degenerate. This degeneration of the intervertebral discs may lead to degenerative disc disease. Degenerative disc disease of the spine is one of the most common conditions causing pain and disability in our population. When a disc degenerates, the nucleus dehydrates. When a nucleus dehydrates, its ability to act as a cushion is reduced. Because the dehydrated nucleus is no longer able to bear loads, the loads are transferred to the annulus and to the facet joints. The annulus and facet joints are not capable of withstanding the applied compression and torsional loads, and as such, they gradually deteriorate. As the annulus and facet joints deteriorate, many other effects ensue, including the narrowing of the interspace, bony spur formation, fragmentation of the annulus, fracture and deterioration of the cartilaginous end plates, and deterioration of the cartilage of the facet joints. The annulus and facet joints lose their structural stability and subtle but pathologic motions occur between the spinal bones.

As the annulus loses stability it tends to bow out and may develop a tear allowing nuclear material to extrude. Breakdown products of the disc and facet joint, including macroscopic chunks, microscopic particles, and noxious biochemical substances build up. These breakdown products stimulate sensitive nerve endings in and around the disc, producing low back pain and sometimes, sciatica. Affected individuals experience muscle spasms, reduced flexibility of the low back, and pain when ordinary movements of the trunk are attempted.

Degenerative disc disease is irreversible. In some cases, the body will eventually stiffen the joints of the motion segment, effectively re-stabilizing the discs. Even in the cases where re-stabilization occurs, the process can take many years and patients often continue to experience disabling pain. Extended painful episodes of longer than three months often leads patients to seek a surgical solution for their pain.

Several methods have been devised to attempt to stabilize the spinal motion segment. Some of these methods include: heating the annular region to destroy nerve endings and strengthen the annulus; applying rigid or semi-rigid support members on the sides of the motion segment or within the disc space; removing and replacing the entire disc with a non-flexible, articulating artificial device; removing and replacing the nucleus; and spinal fusion involving permanently fusing the vertebra adjacent the affected disc.

Until recently, spinal fusion has generally been regarded as the most widely used treatment to alleviate back pain due to degenerative disc disease. While this treatment is effective at relieving back pain, all discal motion is lost in the fused spinal motion segment. The loss of motion in the affected spinal segment necessarily limits the overall spinal mobility of the patient. Ultimately, the spinal fusion places greater stress on the discs adjacent the fused segment as these segments attempt to compensate for lack of motion in the fused segment, often leading to early degeneration of these adjacent spinal segments.

Current developments are focusing on treatments that can preserve some or all of the motion of the affected spinal segment. One of these methods to stabilize the spinal motion segment without the disadvantages of spinal fusion is total disc replacement. Total disc replacement is a highly invasive and technically demanding procedure which includes removing the cartilaginous end plates between the vertebral bone and the disc, large portions of the outer annulus and the complete inner nucleus. If the entire disc is removed, typically an artificial prosthesis is placed in the disc space. Many of the artificial disc prosthesis currently available consist of a soft polymer to act as the nucleus. The soft polymer is interposed between two metal plates that are anchored or attached to the vertebral endplates. A summary of the history of early development and designs of artificial discs is set forth in Ray, “The Artificial Disc: Introduction, History and Socioeconomics,” Chpt. 21, Clinical Efficacy and Outcome in the Diagnosis of Low Back Pain, pgs. 205-225, Raven Press (1992). Examples of these layered total disc replacement devices are shown, for example, in U.S. Pat. Nos. 4,911,718, 5,458,643, 5,545,229 and 6,533,818.

These types of artificial total discs have several disadvantages. First, because the artificial disc prosthetics are relatively large, they require relatively large surgical exposures to accommodate their insertion. The larger the surgical exposure, the higher the chance of infection, hemorrhage or even morbidity. Also, in order to implant the prosthetic, a large portion of the annulus must be removed. Removing a large portion of the annulus reduces the stability of the motion segment, at least until healing occurs around the artificial disc. Further, because the devices are constructed from rigid materials, they can cause serious damage if they were to displace from the disc space and contact local nervous or vascular tissues. Another disadvantage is that rigid artificial disc replacements do not reproduce natural disc mechanics. Finally, relative movement between the hard surfaces of the metal plates of many artificial discs and the vertebral bone will tend to cause erosion of the vertebral endplates. Such endplate erosion can lead to instability, subsidence, and/or neurological or vascular damage.

An alternative to total disc replacement is nuclear replacement. Like the artificial disc prosthetics, these nuclear replacements are also inert, somewhat flexible, non-biological prosthetics. The procedure for implanting a nuclear replacement is less invasive than the procedure for a total disc replacement and generally includes the removal of only the nucleus and replacement of the nucleus with a prosthetic that may be malleable and provide cushioning that mimics a natural disc nucleus. Examples of the prosthetics used for nuclear replacement include: the Ray implant (U.S. Pat. Nos. 4,772,287 and 4,904,260), the Bao implant (U.S. Pat. No. 5,192,326), the Sulzer spiral implant (U.S. Pat. No. 5,919,235), and the Replication Medical implant (U.S. Pat. No. 6,726,721).

Nuclear replacements are intended to more closely mimic natural disc mechanics. To that end, some nuclear replacements utilize hydrogel because of its water imbibing properties. Hydrogel is also used because of its ability to expand in situ to permit a more complete filling of the excavated nuclear cavity. However, there is a trade-off in that the more expansion the hydrogel achieves, the less robust the end product will be. As a result, hydrogel nuclear disc replacements have generally adopted the use of some formed jacket to contain the hydrogel material. For example, the Ray implant as described in U.S. Pat. Nos. 4,772,287 and 4,904,260 consists of a block of hydrogel encased in a plastic fabric casing. The Bao implant as described in U.S. Pat. No. 5,192,326 consists of hydrogel beads enclosed by a fabric shell. While the use of a jacket can result in better structural integrity and less potential extravasation of hydrogel material outside of the nucleus, there is a tendency for jacket encased hydrogel materials to produce a lateral expansive force against the annulus. Another problem with this approach is that the hydrogel material can become too hard for the desired stress response curve of a replacement disc.

Another approach to nucleus replacement involves implantation of a balloon or other container into the nucleus that is filled filling it with a biocompatible material that hardens in situ. Examples of this in situ approach to nucleus replacement include U.S. Pat. Nos. 5,549,679 and 5,571,189. One of the problems with this approach is that the chemical hardening process is exothermic and can generate significant amounts of heat that may cause tissue damage. In addition, there is a possibility that the balloon may rupture during expansion, causing leakage of material out of the bone cavity, which may cause undesirable complications.

Another technique for nucleus replacement involves implanting a multiplicity of individual support members one at a time in the disc space until the cavity is full. Examples of this approach include U.S. Pat. Nos. 5,702,454 and 5,755,797. Because each of the individual components is relatively small, there is a possibility that one or more beads or support members will extrude out of the cavity.

From a mechanical perspective, this technique is limited in the ability to produce consistent and reproducible results because the location and interaction of the multiplicity of beads or support members is not controlled and the beads or support members can shift during and after implantation.

Accordingly, there is a need for a nuclear prosthesis that may be inserted using a minimally invasive procedure and that mimics the characteristics of a natural disc.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for repairing a damaged intervertebral disc nucleus in a minimally invasive manner with a modular disc prosthesis. The modular disc prosthesis preferably comprises at least three modular segments and at least two rails that operably connect adjacent modular segments. In one embodiment, each modular segment includes a harder inner portion and a softer outer portion. Preferably, the rails operate to slidably connect and interlock adjacent modular segments. A stem portion of the rails that extends outside the periphery of the body of the prosthesis is removable after implantation such that the modular segments form an implanted unitary device that closely mimics the geometry of the disc nucleus cavity.

In one embodiment, a modular disc prosthesis that is adapted to be implanted in an evacuated disc nucleus space includes at least three modular segments each having a width. The first modular segment has a first rail extending at least partially along one side of the width and beyond a periphery of the first modular segment. The second modular segment is slidably connected to the first rail on one side of the width and has a second rail extending at least partially along another side of the width and beyond a periphery of the second modular segment. The third modular segment is slidably connected to the second rail on one side of the width. The prosthesis has an expanded position in which the modular segments are extended along the first and second rails and positioned in a generally end to end configuration spaced apart by the rails prior to implantation. The prosthesis also has an implanted position in which the modular segments are positioned in a generally side by side configuration that defines a unitary body having a generally continuous periphery that generally corresponds to the evacuated nucleus disc space with at least a portion of the rails extending beyond the periphery of the body.

Preferably, each modular segment comprises an inner portion and an outer portion. The inner portion includes structure that mates with one of the rails. The outer portion substantially surrounding the inner portion except for the side having one of the rails and the side having structure that mates with one of the rails. In one embodiment, the inner portion of each modular segment and the outer portion of each modular segment are made of polymers of different durometers. Preferably, the inner portion of each modular segment has a compressive modulus from about 70-100 Mpa and the outer portion of each modular segment has a compressive modulus from about 6-20 Mpa. The use of a harder inner portion and softer outer portion as part of an integrated unitary implanted device permits the modular prosthesis of the present invention to more closely mimic the stress response of a biological disc nucleus while simultaneously permitting effective operation of the slidable relationship between adjacent ones of the modular segments.

In one embodiment, locking features are provided to ensure that the modular disc prosthesis is a unitary device both before and after insertion. To prevent the device from being separated prior to insertion, locking features may be provided on the rigid rails to prevent modular segments from being slid back off of the rails. This ensures that each modular segment is connected in its proper position and in the proper order. In addition, locking features may be provided on the modular segments to lock them together upon insertion. This prevents individual segments from dislocating from the assembled prosthetic and migrating within the annulus.

Another aspect of the present invention comprises a method for implanting a modular disc prosthesis. Because the modular disc prosthesis may be implanted one segment at a time, a hole made in the annulus for implantation of the prosthesis may be a fraction of the size of the device in its final assembled form. The modular disc prosthesis is introduced into the patient's intervertebral space through an access tube and the first modular segment is inserted into the disc nucleus space through the small hole in the annulus. The second modular segment is then slid up the first rigid rail using a pushing tool and into the disc nucleus space until the second modular segment interlocks with the first modular segment. The tail stem of the first rigid rail is then severed from the device. In one embodiment, the tail stem of the rigid rails may be removed by a cutting mechanism provided preferably as part of the distal end of the pushing tool. Subsequent modular segments are slid up the adjoining rigid rail with the pushing tool into the disc nucleus space and then interlocked with the previously inserted modular segment in a similar manner. Once all of the modular segments have been inserted and all of the tail stems severed, the modular disc prosthesis is fully inserted into the patient's disc nucleus space and assembled and the access tube and pushing tool may be withdrawn and the access hole is closed up.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a modular disc prosthesis according to the preferred embodiment of the present invention in its inserted configuration.

FIG. 2 is a top view of a modular disc prosthesis according to the preferred embodiment of the present invention prior to insertion.

FIG. 3 is a perspective view of a modular disc prosthesis according to an alternate embodiment of the present invention prior to insertion.

FIG. 4 is a perspective view of a modular disc prosthesis according to an alternate embodiment of the present invention at a first stage of insertion.

FIG. 5 is a perspective a view of a modular disc prosthesis according to an alternate embodiment of the present invention at a second stage of insertion.

FIG. 6 is a perspective view of a modular disc prosthesis according to an alternate embodiment of the present invention at a final state of insertion.

FIG. 7 is a partial perspective view of a portion of a modular disc prosthesis according to an embodiment of the present invention.

[FIG. 8-xx are views of an insertion tube and pushing tool for use in accordance with one embodiment of the present invention.]

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, there can be seen a cross-sectional view of a modular disc prosthesis 100 according to the preferred embodiment of the present invention as configured once inserted into the body. In this embodiment, modular disc prosthesis 100 comprises first 102, second 104, third 106, fourth 108, and fifth 109 modular segments. Preferably, each modular segment 102, 104, 106, 108, 109 comprises a soft outer portion 102 a, 104 a, 106 a, 108 a, 109 a and a hard inner portion 102 b, 104 b, 106 b, 108 b, 109 b.

In a preferred embodiment, hard inner portions 104 b, 106 b and 108 b have an I-beam cross-sectional shape that optimizes flexibility and strength of the hard inner portions. Alternatively, hard inner portions 104 b, 106 b, 108 b, can have a uniformly shaped cross-sectional area to reduce any differences in compressibility of the modular disc prosthesis 100 across the surface area in order to minimize the potential for stress risers to be created in the interface between the outer surface of the modular disc prosthesis 100 and the inner surfaces of the disc space cavity. It will be recognized that various cross-sectional shapes of hard inner portions 102 b, 104 b, 106 b, 108 b and 109 b can be utilized in accordance with the present invention and that the cross-sectional shapes of the hard inner portions does not need to be symmetric.

Hard inner portion 102 b of first modular segment 102 includes first segment interlocking portion 116. Hard inner portion 104 b of second modular segment 104 includes second segment interlocking portion 118 and a first slot 128. Hard inner portion 106 b of third modular segment 106 includes third segment interlocking portion 120 and a second slot 130. Hard inner portion 108 b of fourth modular segment 108 includes fourth segment interlocking portion 121 and a third slot 132. Hard inner portion 109 b of fifth modular segment 109 includes a fourth slot 133.

In the preferred embodiment, rails 110, 112, 114, 115 have a noncircular cross-sectional shape, although it will be understood that other cross-section shapes could be utilized and that there is no requirement that all of the rails have similar cross-sectional shapes. It has been found that the noncircular cross-sectional shape as shown (corresponding mating C and sideways T cross-sectional shapes) provides for better alignment of the modular segments and supports larger insertion forces along the axis of the rail.

It will be understood that in a preferred embodiment, the rails 110, 112, 114, 115 of the present invention have a non-uniform cross-sectional aspect ratio in terms of the height and width of the rail. Preferably, the rails have a relative rigidity along a longitudinal axis of the rail in a dimension of the height of the rail that is greater than a width of the rail, whereas in a dimension transverse to the width of the rail the relative rigidity of the rail permits a greater degree of flexibility such that succeeding modular segments can be moved laterally with respect to one another in the expanded position without deforming the rails. [Insert preferred ranges of dimensions of height and width of the rails]. This differential rigidity in the two dimensions transverse to the longitudinal axis of the rail is important in permitting effective and efficient sliding operation of the adjacent modular segments.

Referring to FIG. 2, there can be seen a portion of a modular disc prosthesis 100 according to the preferred embodiment of the present invention prior to insertion into the disc nucleus space. Note that in FIG. 2, modular disc prosthesis 100 is depicted showing only the hard inner portion 102 b, 104 b, 106 b, 108 b, 109 b of each modular segment 102, 104, 106, 108, 109 for convenience of illustration. However, in the preferred embodiment of the invention each modular segment would also have soft outer portion as described above and shown in FIG. 1.

In alternate embodiments, modular disc prosthesis may comprise greater or fewer numbers of modular segments and rails, so long as there are at least three modular segments and two rails. For example, FIG. 3 depicts a modular disc prosthesis 200 having four modular segments and three rails.

Prior to insertion, modular disc prosthesis 100 further includes first 110, second 112, third 114, and fourth 115 rails. First modular segment 102 is rigidly attached to first rail 115 at first segment interlocking portion 116. Second modular segment 104 is slidably attached to first rail 110 at first slot 128 and rigidly attached to second rail 112 at second segment interlocking portion 118. Third modular segment 106 is slidably attached to second rail 112 at second slot 130 and rigidly attached to third rail 114 at third segment interlocking portion 120. Fourth modular segment 108 is slidably attached to third rail 114 at third slot 132 and rigidly attached to fourth rail 115 at fourth segment interlocking portion 121. Fifth modular segment 109 is slidably attached to fourth rail 115 at fourth slot 133.

As shown in FIG. 2 and FIG. 3, each rail 110, 112, 114 and 115 or 210, 212 and 214 include a stem portion that extends beyond a periphery of the body of the prosthesis 100, 200, respectively. Preferably these stem portions are long enough to permit access into the evacuated disc nucleus space such that one modular segment can be positioned inside the evacuated disc nucleus space while the next modular segment on the rail is still outside of the body. In an exemplary embodiment, the length of these stem portions ranges between [insert range of lengths of stem portions].

As shown in the alternate embodiment of FIG. 3, each rail 210, 212, 214 may further include a retaining portion 222, 224, 226 to keep the device from being separated prior to insertion. The retaining portions 222, 224 and 226 are configured to prevent the corresponding rail 210, 212 and 214 from being slid off the rail. The retaining portions may be molded into the rails or may be separate pieces or deformations of the rails added during the manufacture of the device.

The preferred embodiment is a unitary prosthesis that comes packaged, sterile, and ready for implantation at the surgical site. Since the device is fully preformed and delivered as a unitary implant, the device is under direct operator control until the prosthetic disc nucleus is completely formed. This unitary design limits the need for the surgeon to determine how the cavity should be filled and assures that the components order of insertion and connection cannot be mixed up. The ability to predetermine the size of modular disc prosthesis also allows for the nucleus cavity to be more completely filled and provides a greater degree of control over the uniformity of the stress response of the implant as compared to other kinds of minimally invasive implants. In this regard, it will be understood that the modular disc prosthesis 100 of the present invention may be provided in a variety of different final assembled volumes and shapes to correspond to different sizes and shapes of different evacuated disc cavities.

Modular disc prosthesis is introduced through an access tube that is inserted partially into the disc nucleus space. As shown in FIG. 8, access tube is at least 3 inches long and preferably about 6 inches long. Modular segments may be transposed along rails by means of a pushing tool as shown in FIG. 9. It should be noted that although the insertion of modular disc prosthesis is described in relation to a preferred five-segment embodiment or an alternate four-segment embodiment, embodiments having any other number of segments would be inserted in a similar fashion.

Referring again to FIG. 3, there can be seen a modular disc prosthesis 200 prior to insertion into the body. Upon inserting the access tube into the disc nucleus space, first modular segment 202 is inserted through the tube and into the disc space. Upon complete insertion of first modular segment 202, modular disc prosthesis 200 is moved centrally and second modular segment 204 is slid along the first rail 210 into the disc nucleus space onto first segment interlocking portion 216 until it is flush with first modular segment 202. This stage of insertion is depicted in FIG. 4. A stem portion of first rail 210 is then removed and modular disc prosthesis 200 is moved centrally again.

Third modular segment 206 is then slid down the second rail 212 and into the disc nucleus space onto second segment interlocking portion 218 until it is flush with second modular segment 204. This configuration is shown in FIG. 5. A stem portion of second rail 212 is then removed and modular disc prosthesis 200 is moved centrally. Fourth modular segment 208 is slid along third rail 214 into the disc nucleus space and onto third segment interlocking portion 220 until it is flush with the other modular segments 202, 204, 206. Finally, a stem portion of third rail 214 is then removed. This final implanted configuration of modular disc prosthesis 200 with all modular segments aligned and locked together is shown in FIG. 6. Modular disc prosthesis 200 is sized and shaped to conform to the geometry of the disc nucleus cavity.

In an alternate embodiment, a keystone approach can be used to insert modular disc prosthesis such that the last modular segment inserted into the disc nucleus space is not one of the outside segments. Instead, the outside segments can be the first two segments inserted. This creates a bilateral expansion force as the remaining segments are inserted between the two outside segments. This helps make a tighter fit within the disc nucleus space than does the asymmetric lateral force imparted when the segments are implanted sequentially.

The stem portions of rails 110, 112, 114, 115 that extend beyond the periphery of the body of the modular disc prosthesis 100 can be removed by many different techniques. As shown in FIG. 9, pushing tool may be provided with a cutting mechanism that can remove the stem portions of the rails. Cutting mechanism may be a pair of fixed blades located on the distal end of pushing tool. In this embodiment, the cutting blades would act as a cutting wheel in which a turning of the handle connected to the blades causes the blades to circumscribe the rail. Alternatively, the cutting mechanism can be a clamping means that removes the rails through twisting or pinching. Stem rails may also be cut off with any other sharp instrument.

In another embodiment, the stem portions of the rails may be provided with a perforation at the junction with each modular segment such that they can be torn, broken, twisted, or more easily cut off. Cutting may also be accomplished with a wire loop provided to the part. Additionally, heat, laser, or any other local energy source can be used to accomplish the separation. One of skill in the art will recognize that numerous alternative means exist whereby stem rails can be severed from modular disc prosthesis.

Alternatively, modular disc prosthesis may be implanted using an anterior lateral approach. An anterior lateral approach allows for a larger insertion opening to be used while still being minimally invasive. [Insert any further description of this alternate anterior lateral surgical technique.]

During insertion, slots 128, 130, 132, 133 slide along the stem portions of rails 110, 112, 114, 115 and onto segment interlocking portions 116, 118, 120, 121. Slots 128, 130, 132, 133 and segment interlocking portions 116, 118, 120, 121 may be provided with locking features to prevent separation of modular segments 102, 104, 106, 108, 109. Locking features, such as a series of barbs or studs, may be provided such that once a slot is slid onto a segment interlocking portion, it cannot be slid back off of it. A ratchet and pawl may also be used to lock modular segments together. A ratchet release tool may also be provided in case separation of modular segments is desired once they are locked together.

One example of these locking features is depicted in FIG. 7. Hard inner portion 304 b of each modular segment 304 is provided with a pair of depressible projections 334 on segment interlocking portion 318 and a complementary pair of apertures 336 on slot 328. When slot of a first modular segment is slid onto segment interlocking portion of a second modular segment, projections are depressed. When apertures of the first modular segment are positioned over projections of the second modular segment, the projections pop through apertures, locking the modular segments relative to one another. Modular segments may be separated by depressing the projections and sliding the first modular segment back off of the second modular segment.

Alternatively, free movement of modular segments 102, 104, 106, 108, 109 along rails 110, 112, 114, 115 may be allowed until insertion in the body. It will be understood that, depending upon the material configuration of the modular prosthetic 100 and the interface fit, segment interlocking portions 116, 118, 120, 121 may swell due to hydration to lock in the final configuration. The feature may be used alone or in combination with a mechanical locking feature. Alternative methods of locking modular segments together will be appreciated by those skilled in the art.

In the preferred embodiment, modular disc prosthesis 100 is molded from elastomeric biomaterials, preferably polyurethane. Stem rails 110, 112, 114, 115 and hard inner portions 102 b, 104 b, 106 b, 108 b, 109 b are made from a hard durometer polyurethane, such as a polyurethane with shore hardness 55D or above and compressive modulus of 70 to 100 MPa. Soft outer portions 102 a, 104 a, 106 a, 108 a, 109 a are made from a soft durometer polyurethane, such as a polyurethane with a shore hardness ranging from 55D to 18A and a compression modulus between 6 and 20 MPa.

In the preferred embodiment, the two different durometer polyurethanes are co-polymerized to create a chemical bond between the two portions of each modular segment 102, 104, 106, 108, 109. In alternate embodiments other polymers such as PEEK, polyethylene, silicones, acrylates, nylon, polyacetyls, and other similar engineering polymers may be used for the hard inner portions or the soft outer portions. For a more detailed description of a preferred embodiment of the multi-durometer polymer compositions of the present invention, reference is made to the previously identified co-pending application entitled, “[fill in final title]”.

In an alternate embodiment, the stem of the tails may be molded from a harder durometer material than soft outer portion and hard inner portion of modular segments. Utilizing this approach allows the rails to be extruded, rather than molded as part of the modular segments. A bond joint can then be made with the hard inner portion external to the periphery of the modular segments to form the unitary design. Extruding the stem portions of the tails makes modular disc prosthesis easier and less expensive to manufacture than a completely molded product.

In the preferred embodiment, the soft outer portion of modular disc prosthesis is deformable in response to normal physiological forces of 30 to 300 pounds. Because of this increased deformability, the prosthesis produces little impingement on the end plates of the intervertebral disc. As a result, the end plates do not flatten out over time and conform to the contours of the implant as is the case with many metal/polymer disc replacement implants.

In an alternate embodiment, outer portion of modular disc prosthesis may be modified to provide for elution of medicants. Such medicants can include analgesics, antibiotics, or bioosteologics such as bone growth agents. The solid polymer outer portion of modular disc prosthesis provides for better and more controllable elution rates than hydrogel materials can. [Insert further discussion of elution, including whether there could be another polymer layer on the very outside that would serve as an “elution” layer”]

Various modifications to the disclosed apparatuses and methods may be apparent to one of skill in the art upon reading this disclosure. The above is not contemplated to limit the scope of the present invention, which is limited only by the claims below. 

1. a modular disc prosthesis that is adapted to be implanted in an evacuated disc nucleus space, the prosthesis having an expanded position before implantation and an implanted position after implantation, the prosthesis comprising: a plurality of modular segments that, when in the implanted position, are arranged side by side from a first end of the prosthesis to a second end of the prosthesis, the plurality of modular segments being configured for assembly in sequence from the first end to the second end of the prosthesis and cooperating with each other when in the implanted position to define a unitary body having a generally continuous periphery that generally corresponds to the evacuated nucleus disc space, a first of the plurality of modular segments being integral with a first rail, a second of the plurality of modular segments being integral with a second rail, the second modular segment being slidably connected to the first rail, and a third of the plurality of modular segments being slidably connected to the second rail, wherein each of the first and second rails include an extended portion that extends beyond the generally continuous periphery of the unitary body of the implanted position, the extended portion of the first rail being selectively removable from the first of the plurality of modular segments once the second of the plurality of modular segments is in the implanted position, and the extended portion of the second rail being selectively removable from the second of the plurality of modular segments once the third of the plurality of modular segments is in the implanted position, wherein both the second and third modular segments are separated from each other and slidably connected to the extended portions of the first and second rails respectively when the prosthesis is in the expanded position to guide the second and third modular segments respectively to the implanted position.
 2. The modular disc prosthesis of claim 1, wherein a third rail is integral with the third modular segment, the third rail including an extended portion that extends beyond the generally continuous periphery, wherein the modular disc prosthesis further comprises a fourth modular segment slidably connected to the extended portions of the third rail when the prosthesis is in the expanded position to guide the fourth modular segment to the implanted position, the extended portion of the third rail being selectively removable from the third of the plurality of modular segments once the fourth of the plurality of modular segments is in the implanted position.
 3. The modular disc prosthesis of claim 1, wherein the extended portions of the first and second rails each have a rigidity in a first transverse direction relative to the longitudinal axis of the respective rail that differs substantially from a rigidity in a second transverse direction relative to the longitudinal axis of the respective rail.
 4. The modular disc prosthesis of claim 1, wherein each of the second and third modular segments comprise an inner portion and an outer portion, each outer portion being operatively coupled with a portion of the respective inner portion, the inner portions of the second and third modular segments being slidably engaged with the first and second rails, respectively.
 5. The modular disc prosthesis of claim 4, wherein the inner portion of the second and third modular segments and the outer portion of the second and third modular segments are made of polymers of different durometers.
 6. The modular disc prosthesis of claim 1, wherein the first, second and third modular segments each comprise a moldable polymer material and the extended portion of each rail comprises an extrudable polymer material.
 7. A modular disc prosthesis that is adapted to be implanted in an evacuated disc nucleus space, the prosthesis having an expanded position before implantation and an implanted position after implantation, the prosthesis comprising: a plurality of modular segments that, when in the implanted position, are arranged side by side in generally one of a medial-to-lateral orientation and a posterior-to-anterior orientation, the plurality of modular segments being configured for assembly in sequence from a first end to a second end of the prosthesis along the orientation, a first of the plurality of modular segments being operatively coupled with first means for slidably guiding a second modular segment along a first longitudinal axis when the second modular segment is manipulated from the expanded position to the implanted position, the second modular segment being slidably connected to the first means for slidably guiding the second modular segment along the first longitudinal axis, the second modular segment being operatively coupled with second means for slidably guiding a third modular segment along a second longitudinal axis when the third modular segment is manipulated from the expanded position to the implanted position, the third modular segment being slidably connected to the second means for slidably guiding the third modular segment along the second longitudinal axis, wherein both the second and third modular segments are separated from each other and are slidably connected to the first and second means for slidably guiding, respectively, when in the expanded position, the modular segments being in contact with each other when in the implanted position and defining a unitary body having a generally continuous periphery that generally corresponds to the evacuated nucleus disc space, the first means for slidably guiding the second modular segment extending beyond the generally continuous periphery and along the first longitudinal axis when the first modular segment is in the implanted position, the first means for slidably guiding the second modular segment being selectively detachable from the first modular segment once the second modular segment is in the implanted position, the second means for slidably guiding the third modular segment extending beyond the generally continuous periphery and along the second longitudinal axis when the second modular segment is in the implanted position, the second means for slidably guiding the third modular segment being selectively detachable from the second modular segment once the third modular segment is in the implanted position.
 8. The modular disc prosthesis of claim 1, wherein the first and second rails are substantially parallel.
 9. The modular disc prosthesis of claim 7 wherein the third modular segment is operatively coupled with third means for slidably guiding a fourth modular segment along a third longitudinal axis when the fourth modular segment is manipulated from the expanded position to the implanted position, the fourth modular segment being slidably connected to the third means for slidably guiding the fourth modular segment along the third longitudinal axis, the third means for slidably guiding the fourth modular segment extending beyond the generally continuous periphery and along the third longitudinal axis when the third modular segment is in the implanted position, the third means for slidably guiding the fourth modular segment being selectively removable once the third modular segment is in the implanted position.
 10. A modular disc prosthesis that is adapted to be implanted in an evacuated disc nucleus space, the prosthesis having an expanded position before implantation and an implanted position after implantation, the prosthesis comprising: a plurality of modular segments that cooperate with each other when in the implanted position to define a unitary body having a generally continuous periphery that generally corresponds to the evacuated nucleus disc space, a first of the plurality of modular segments being integral with a first rail, the first rail including an extended portion that extends beyond the generally continuous periphery of the unitary body of the implanted position, the extended portion of the first rail having a length, a second of the plurality of modular segments slidably connected to the first rail and being integral with a second rail, the second rail including an extended portion that extends beyond the generally continuous periphery of the unitary body of the implanted position, and a third of the plurality of modular segments being slidably connected to the second rail, wherein both the second and third modular segments are separated from each other and slidably connected to the extended portions of the first and second rails respectively when the prosthesis is in the expanded position to guide the second and third modular segments respectively to the implanted position, and wherein the third segment is capable of being separated from the first segment by a distance that is greater than the length of the first rail when in the expanded position.
 11. The modular disc prosthesis of claim 10, wherein a third rail is integral with the third modular segment, the third rail including an extended portion that extends beyond the generally continuous periphery, wherein the modular disc prosthesis further comprises a fourth modular segment slidably connected to the extended portions of the third rail when the prosthesis is in the expanded position to guide the fourth modular segment to the implanted position, the extended portion of the third rail being selectively removable from the third of the plurality of modular segments once the fourth of the plurality of modular segments is in the implanted position.
 12. The modular disc prosthesis of claim 10, wherein the extended portions of the first and second rails each have a rigidity in a first transverse direction relative to the longitudinal axis of the respective rail that differs substantially from a rigidity in a second transverse direction relative to the longitudinal axis of the respective rail.
 13. The modular disc prosthesis of claim 10, wherein each of the second and third modular segments comprise an inner portion and an outer portion, each outer portion being operatively coupled with a portion of the respective inner portion, the inner portions of the second and third modular segments being slidably engaged with the first and second rails, respectively.
 14. The modular disc prosthesis of claim 13, wherein the inner portion of the second and third modular segments and the outer portion of the second and third modular segments are made of polymers of different durometers.
 15. The modular disc prosthesis of claim 10, wherein the first, second and third modular segments each comprise a moldable polymer material and the extended portion of each rail comprises an extrudable polymer material.
 16. The modular disc prosthesis of claim 10, wherein the first and second rails are substantially parallel.
 17. The modular disc prosthesis of claim 10, wherein the plurality of modular segments are arranged side by side from a first end of the prosthesis to a second end of the prosthesis and are configured for assembly in sequence from the first end to the second end of the prosthesis.
 18. The modular disc prosthesis of claim 10, wherein the extended portion of the first rail is selectively removable from the first of the plurality of modular segments once the second of the plurality of modular segments is in the implanted position. 